Chromitite petrogenesis in the mantle section of the Ballantrae Ophiolite Complex (Scotland)

Abstract

Podiform chromitites from the Ballantrae Ophiolite Complex (BOC), NW Scotland, are examined to investigate their petrogenesis and elucidate the nature of melt percolation in the supra-subduction zone oceanic mantle more generally. The mantle portion of the BOC comprises two petrologically distinct serpentinite belts, whose differences have previously been attributed to different degrees of upper mantle melt extraction. Chromitite occurs in each of the northern and southern serpentinite belts, at Pinbain Bridge and Poundland Burn, respectively. Field relationships suggest that chromitites were formed by melt-rock reaction in channel-like conduits in the upper mantle. Chromitite Cr-spinel compositions from the two localities show marked differences to one another, with the Pinbain Bridge chromitite Cr-spinels being characterised by relatively high Cr# [Cr/(Cr+Al); 0.62-0.65] and lower abundances of certain trace elements (e.g., Ti, Ga, V), whereas the Poundland Burn chromitite Cr-spinels exhibit relatively low Cr# (0.44-0.46) and higher concentrations of these trace elements. The contrasting Cr-spinel compositions are used to estimate parental magma compositions for the chromitites; the Pinbain Bridge chromitites crystallised from magmas resembling arc tholeiites whereas MORB-like magmas were involved in formation of the Poundland Burn chromitites. While it is possible that this dichotomy points to early derivation of the BOC at a MORB spreading centre, with subsequent processing in a supra-subduction zone, we suggest that the differences reflect melt extraction from different parts of an evolving subduction zone, such that the MORB-like magmas were generated in a back-arc setting. This interpretation

Trace Elements in Chromian Spinels from Four Siberian Kimberlites

“…A role of COHS-bearing fluids, which are expected in sub-cratonic mantle environments related to kimberlite genesis, could therefore also be a controlling factor for the observed variability of Zn in both Xen and Chr spinels of this study. It is worth noting that Zn 2+ and Co 2+ in the tetrahedral coordination have a very similar ionic radius and consequently T-O distances (1.98 and 1.96 Å [39]; 1.96 Å for both [40]; 1.960 and 1.972 Å, [41]), which may explain the systematic correlations between these two elements in Cr-spinel from different host lithologies (this work, see Supplementary Figure S1; ophiolitic chromite [42][43][44]; magmatic and metamorphic chromite [44] Cr-spinel in mantle xenoliths [18]).…”

The Österplana Fossil Meteorites and… What Else? Terrestrial Cr-Spinels and Zircons in the Ordovician Limestones of the Thorsberg Quarry (Sweden)

“…This value is particularly interesting because, in meteoritic spinels, the Fe 3+ is virtually absent, so that it allows the possibility to discriminate between terrestrial and extraterrestrial occurrences. [41]; red dotted lines: Cr-spinels from Shetland ophiolites [42]; red dot and dash lines: ferrian-chromite rims of Cr-spinels from Shetland ophiolites [42]; blue dotted lines: Cr-spinels from Leka ophiolites [43]; blue dot and dash lines: ferrian-chromite rims of Crspinels from Leka ophiolites [43]. Fields for Arc, OIB (ocean island basalts), BABB (back arc basin Basalts), and MORB (mid ocean ridge basalts) after [12,19].…”

“…Anyway, they differ from the closest Alaskan complexes of the Urals, that, by the way, are also younger, showing a comparable Cr# in the range 0.60-0.85, but lower Fe# in the range 0.2-0.8 [44,45]. When inserted into the ternary Cr-Fe 3+ -Al diagram, they perfectly fall into the field of alkali basalts, lamprophyres, [41]; red dotted lines: Cr-spinels from Shetland ophiolites [42]; red dot and dash lines: ferrian-chromite rims of Cr-spinels from Shetland ophiolites [42]; blue dotted lines: Cr-spinels from Leka ophiolites [43]; blue dot and dash lines: ferrian-chromite rims of Cr-spinels from Leka ophiolites [43]. Fields for Arc, OIB (ocean island basalts), BABB (back arc basin Basalts), and MORB (mid ocean ridge basalts) after [12,19].…”

Melt Percolation, Melt-Rock Reaction and Oxygen Fugacity in Supra-Subduction Zone Mantle and Lower Crust from the Leka Ophiolite Complex, Norway

“…Taitao, Chile (Schulte et al 2009), parts of the Italian Ligurian Complex (e.g., Tribuzio et al 2000) and Masirah, Oman (Rollinson, 2017). However, although classifications such as these, based on detailed consideration of magma petrogenesis and tectonic context, are useful, it is also important to remember that individual ophiolite complexes may preserve evidence of episodes of magmatism in differing settings (e.g., Melcher et al, 1997;Schulte et al, 2009;Goodenough et al, 2014;O'Driscoll et al, 2015;Derbyshire et al, 2019).…”